Electrical components and methods



Aug. 5, 1958 R. l. HARRISON ELECTRICAL COMPONENTS AND METHODS Filed Aug.24, 1954 Unite ELECTRICAL COMPONENTS AND METHODS Application August 24,1954, SerialNo. 451,72

10 Claims. (Cl. 29-2514) The present invention relates to electricalcomponents, such as encased and supported impedance andsignaltransmission elements, more particularly to an improved processforfabricating such components and to resultant electrical devicesexhibiting enhanced mechanical and electrical properties. Specifically,the present invention is concerned with an improved method ofmanufacturing traveling wave tubes and like =devices incorporating ahelical wire conductor and with traveling wave-devices having improvedcharacteristics, prominently reliability and ruggedness.

The impedance and signal-transmission characteristics of coils, helicesand the like, useful as circuit components and asparts of electrical andelectronic devices, frequently depend upon-turn distribution and/or thematerial surrounding and between turns. For example, the inductance of acoil and the signal-transmission properties of a helical conductorchange with variations in intertum spacing and the proximity ofdielectricmaterial to the respective turns.

Accordingly, it is an object of the present invention to provide animproved method of fabricating electrical components. To advantage,encased and supportedcoils, helices and the like may be constructedhaving ta (fixed turn distribution-and desirable electrical andmechanical properties.

:In one form, the inventionmay be best demonstrated as applied to thewell-known traveling wave tube which is essentially an electron beamtube employing an electronjgun. The electron gun, similar to thosecommonly .foundin cathode ray tubes, forms and projects an electron beamaxially within a surrounding wire helical coniductor which is excited bya high frequency input signal at the inputcnd thereof. During travel ofthe input signal :along the helical conductor, there is interaction withthe electron .beam which results inamplificationof a broad band of inputfrequencies available .at the output end of the helical conductor.

Suchknown traveling wave tubes :includean elongated wire helix, encasedwithin a vitreous envelope or tube throughout its length andappropriately .assembled with .input and output coupling devices, aswell as beam .collecting and focusing electrodes. Such helix assemblies,usually -.of relatively small dimensions, are comparatively .longandinclude many turns in relation to thediameter of the helix. Theaccomplishment of entry transfer between the beam and the helix oftennecessitates the .use of T00-t0 800 turns for a helix length ofapproximately 15 inches and helix diameters of the order of of an inch.

The support of such a helix within a vitreous sheath or envelopepresents a rather troublesome problem, :par-

ticularly since the assembly is rather small and hascritical electricalproperties whichmay vary greatly and in a random fashion depending uponthe mounting of the helix. Of primary importance is the fixing of therespective :turns of the helix in relation to each other in accordancewith the established helix pitch, whether uniform or varied. Such fixingof the turn=distribution of the helix should be accomplished withoutadversely-affecting theelectrical properties of the helix, as .byestablishing microwave mismatches or causingjprohibitive'standing waveratios.

Numerous approaches have been suggested win the manufacture oftraveling-wave devices for formingamencased helix having a fixed turndistributionand desirable electrical properties. Among these approacheshas been theformation of aninternal thread on a precisely bored tubing,followed by deposition of metallic materialvin the threads and theremoval of any metallic material .in the spaces between successive turnsof the thread. The inherent complexity of such procedure isapparent,,;particularly when it is considered that the thread .musP-beformed within a long tubing having --a. relatively :smallwinternalcross-section.

Accordingly, it is another object of thejpresent invention to provide anovel method of fabricating traveling wave tubes which obviates one ormore of the aforesaid difiiculties. Specifically, it is within thecontemplation of the invention to provide an improved process formechanically supporting the helix of a traveling wave-device withaparticular turn-distribution and without alter- .ing said turndistribution and/ or adversely .afiectingthe electrical properties ofthe final traveling wave tube'assembly.

As applied to traveling wave tubes, the present invention employs theprinciple of assembling ahelix within a closely conforming vitreoussheath or tubing, -and.-heat softening and shrinking'the vitreous sheathinto supportingrelation with the helix, thus fixing the distributiontofthe helix turns. A helix supported by heat shrinking-bf its vitreoussheathing into supporting relation Withith helix turns has been found tobe exceptionally rugged;

furthersaid :helix exhibits electrical characteristics compatible withproper functioning of traveling wave devices.

In accordance with an illustrative process -.demonstrating .features ofthe present invention, a wire-helix :is

wound upon a mandrel andpreferably assembledwithdevices for input andoutput coupling of wave energy to the helix. 'The subassembly of thewoundwhelix and coupling devices is then placed within and axially obaglass tube having a bore diameter closely-conformingto the outerdiameter of the helix. Theassembly .of the helix and the sheathing ortube is elevated to .a'tempera ture appropriate to heat soften andshrink the gglass-into contact with the respective turns of the helix.:Bycontrol over the shrinking rate, relatively rigid tolerances may bemaintained for the degree of contact betweenthc ,g'lass 'of the tube andthe helix turns, as well as tthepene- 'tration of glass between therespective turns of the helix.

Preferably the .process is carried out with the mandrel substantiallyfilling out the helix :space and theassembly continuously rotated aboutthe common of 'themandrel and the vitreous tube suchthat'the tube doesnot bow throughout its length due to gravitational forces.

Additionally, the heat shrinking is preferably-carried out w'i'th'theinterior of the .tube at .a reduced pressure with 'resp'ectto "theexterior 'of'the tube. This'pressure differential lowers the shrinkingtemperature, thus: providing "high viscosity of "the glass at relativelylow *control sen- -sitivity--of-glass sagging.

. he- 'above :as we'llasstill further =objects, features and :advantagesof the present invention will be-sbc'st appreciated by reference :to:the following detailed description of "-a presently preferred processand :articles attainable thereby, when taken in conjunction with:theaaccompanying drawings, wherein: Figure .1 is an elevational'viewwith parts broken'taway, showing a traveling wave tube.assemblyprocesseduin accordance with features of the present invention;

Figure 2 is a longitudinal sectional view, enlarged and foreshortened,showing the assembly of a mandrel having a helix and coupling probleswith a vitreous sheathing assembled prior to a processing in accordancewith the present invention;

. Figure 3 is a sectional view, enlarged and foreshortened, of asheathed helix and probe assembly processed in accordance with thepresent invention;

Figure 4 is a fragmentary sectional view on a greatly enlarged andexaggerated scale showing the contact region between the turns of thehelix and the encasing vitreous sheath.

In Figure'l there is shown an illustrative traveling wave tube whichincludes a vitreous body 12 having an enlarged bulb 14 and an elongatedsheath or tube 16. Within the bulb 14 is the well-known electron gunassembly 18 which is arranged to produce a beam to be directed axiallyalong the envelope or tube 16 and includes the usual filamentary heater,electron-emitting cathode and beam-forming elements. The electrodeassembly within the elongated tube or sheath 16 includes an input probe20, a helix 22 of variable or uniform pitch having one end electricallyconnected to the input probe 20, and an output probe 24 electricallyconnected to the other end of the helix. Beyond the output probe 24 is acollector 26 for the electron beam having an appropriate externalterminal or connection. As is well understood appro priate connectionsare provided for establishing operating potentials and a magnetic fieldto focus the electron beam. with this broadly old traveling wave tube,further descrlption as to the details of the tube construction will bedispensed with except as is necessary for an understanding of thepresent improved method.

Referring now specifically to Figure 2, there is shown U the helix 22which is of tungsten wire wound on a mandrel M of a material having acoefiicient of expansion selected to avoid indenting of the helixmaterial. It has been found that a steel mandrel maybe used successfullywith a wire helix of either tungsten or molydenum having a coppercoating. Preferably, the helix and its mandrel are fired at atemperature of approximately 900 C. fora period of three to four minutesin a protective atmosphere to set the helix turns. probes 20, 24 areassembled with the helix 22, as by spot welding or the like. The mandrelM with the assembled helix 22 and energy-coupling probes 20, 24 isinserted within the vitreous bulb envelope or body 12 which may be of aglass, such as Corning 7052. far as the rate of propagation ofelectromagnetic wave energy along the helix is critically dependent uponthe dielectric medium in the immediate vicinity of the helix 22, theglass of the envelope 12 is selected to have an appropriate dielectricconstant for microwave applications. The envelope 12, illustrated inincomplete state with its opposite ends open, includes the tubing 16 precisely bored in the region coextensive with the helix and probeassembly, the tubing 16 having an inside diameter approximately equal tothe outside diameter of the helix and probe assembly. The envelope 12maybe preliminarily processed to form an attenuated coating thereon inselected regions, as by evaporating a nichrome film onto the inner wallsof the tube 16 as shown in U. S. Patent No. 2,660,690.

Preliminary to processing in accordance with the present invention itmay be further desirable to cold match .the inserted helix and probes toobtain an appropriate radio frequency match and to determine whether theassembly meets other predetermined standards of electrical performance.The cold match is carried out by making appropriate connections to theinput and output ends of the assembly and testing on a radio frequencybench. Acceptable assemblies are then completed by fitting the collector26 on the reduced end m of the In that the present invention is notconcerned The input and output I Insomandrel M, the mandrel M serving toalign the respective probes 20, 24, the helix 22 and the collector 26.

To preclude lengthening and foreshortening of the helix 22 during heattreatment, the probes 20, 24 are secured to the vitreous sheath ortubing 16. This may be accomplished by dimpling the tubing 16 or byapplication of a cold ceramic cement, such as Sauereisen #29 at thelocations 30, 32. The mandrel M is fixed in relation to the envelope orbulb 12 by pinching down on the envelope or blank at its opposite ends,as illustrated at the left in Figure 2 and designated by the numeral 34.The right end of the tube 12 beyond the collector 26 may likewise besecured in relation to the mandrel M by pinching down on the glasstubing. At points beyond the region of support of the tube 16 by themandrel M, it may be desirable to provide extension tubings which have asufiiciently high melting and softening point as compared to the glassof the envelope for convenience in working and to preclude bowing of theassembly.

The assembly of the mandrel M and the envelope 12 is then supportedbetween the chucks C C The chuck C is a vacuum chuck connected to anappropriate exhaust pump, while the chuck C is an ordinary mechanicalchuck. The end of the envelope-mandrel assembly supported in chuck Cwhich may be an extension of a high melting and softening point glass,is appropriately sealed, as by a stopper (not shown). The use of themandrel M for centering the probes 20, 24, the helix 22 and thecollector 26 assures perfect coaxial relation between the respectivecomponents of the final tube assembly; while the use of thelow-expansion cold cement having good electrical properties and capableof withstanding rela' tively high vacuum assures that the helix 22 willnot change length.

The assembly of Figure 2, while under vacuum and rotating, is heated, asby application of a flarne directly thereto or by placing the assemblywithin an oven. The heating, whether in a localized area or throughoutthe length of the assembly, is continued for a period and at atemperature to sofeten and shrink the tubing 16 onto the probe-helixassembly. The shrinking temperature is dependent upon the type ofvitreous material used and on the pressure differential existing betweenthe inner and outer walls of the sealed-off envelope 12. Slow shrinkageis preferred to facilitate accurate control over the depth of sagging orpenetration of the vitreous material between the respective turns of thehelix 22. To this end, the use of a relatively high vacuum and lowershrinking temperatures assures low flow rates of the vitreous materialdue to the high viscosity of the glass. The use of reduced temperaturesbesides providing for easier control over glass sagging, minimizes therisk of oxidizing the metal tube parts as well as fusing copper from theplating of the helix wire onto the adjacent vitreous walls. Such fusingof copper onto the glass walls increases the attenuation of microwavepower and is appreciably reduced by lowering the shrinking temperature.By careful control of the shrinking temperature and by operating atvacuums on the order of one-tenth of an atmosphere or less, it has beenfound possible to obtain uniform shrinkage of the vitreous materialbetween respective turns of the helix 22 to fix the preestablished turndistribution of the helix 22. Although it is preferable to obtain thepressure difierential between the inside and outside walls of theenvelope 12 by drawing a vacuum therein, it is equally within the scopeof the invention to apply pressure externally of the envelope 12.

When the required glass sagging is obtained, the temperature of theassembly is reduced to stop the sagging action. During the reversecycling of the heat-shrunken assembly, it is preferable to continuerotation and operation under vacuum. By slow cooling, it is possible toanneal the tubing with attendant advantages.

The helix and probe assembly within the envelope 12 is then prepared forincorporation into the final traveling wave tube by removing theextension portions of the euvelope by cutting at the parting planes P Pillustrated in'Figure 2. The final helix and probe assembly with themandrel removed is shown in Figure 3. Preliminary to removal of themandrel M, the lead or extension 28 for the collector 26 is sealed tothe envelope, as indicated at 36.

Upon inspection of Figure 4,.it will be appreciated that intermediatethe respective turns of the helix 22, there are integral portions of thetube 16 which provide peripherally-extending andcirciunferentially-extending area contact to the respective turns of thehelix '22. The area contact embraces the outer surface ofthe wire of thehelix throughout the length of the wire. Such embracing arcuate contact,providing a helical groove mating with the outer surface of the wire ofthe helix, contrasts markedly to constructions in which the helix issnugly received in its supporting envelope. The showings of Figures 2 to4 inclusive are exaggerated dimensiona'lly such that the physicalconfigurations attainable in accordance with the present invention maybe fully appreciated. However, a typical traveling wave tube assemblyincludes a helix having an inside diameter of approximately .1 inchwhich helix is fabricated of a tungsten wire of mils diameter and havingan electroplated copper Coating of approximately .5 mil. The outsidediameter of the helix is approximately .11 inch, the helix beingreceived within a precisely bored tubing having aninside diameter of .12inch and an outside diameter .of..210 .inch. A typical assembly includes770 turnsin a length of approximately 15% inches. Approximately .2 milof total sagging, that is .07% of the wirec'ircumference, has been found'to'be sufiicient to secure the'turns of the helix in relation to eachother without significant interference with the electricalcharacteristics of the tube due to the presence of prohibitive amountsof diel'ectric material between the respective turns. Traveling wavetubes with desirable characteristics have been constructed having 3%circumferential wetting of the helix turns by the dielectric material ofthe sheath. The aforesaid dimensions are purely illustrative and are setforth such that the relative proportions of one such typical structuremay be more fully appreciated.

Following processing of the probe and helix assembly, the electron gun18 is placed within the bulb 14 and the remaining electrodes and leadsconnected in accordance with techniques which are well understood.Thereafter the envelope 12 is evacuated and sealed.

The wide variety of modifications as well as further details will beapparent to those skilled in the art. For example, the helix 22 may bewound on a variable pitch throughout selected regions of its length, toadvantage in certain types of microwave applications. Still furtherrefinement may be made in control of the temperature throughout thevarious portions of the helix and probe assembly. For example, in theregions adjacent to the probes 2t 24 more localized heating may berequired in that a large area of heat conduction is provided to thevitreous wall or sheath 16; while in the regions occupied by the helix22, such large areas of heat conduction are not present.

Although the invention has been described as applied to traveling waveamplifying devices, application of the principles herein to manufactureof precise inductance coils and tube electric components is likewiseintended.

Further modifications and varied applications of the foregoing inventionwill occur to those skilled in the art. Accordingly, the appended claimsshould be construed broadly, consistent with the spirit and scope of thepresent invention.

What I claim is:

1. In the manufacture of traveling wave tubes, the steps includingsubassembling a helix and probes concentrically of a mandrel, placingsaid mandrel in a vitreous g t tube, fixing said probes to said=vitreoustube :to prevent elongation and foreshortening of said helix,drawing a vacuum in said vitreous tube, axially rotating the assembly ofsaid mandrel and vitreous tube, and heating said assembly while rotatingto shrink said vitreous tube 'onto said helix .to permanently .fixthehelix turns in place.

2. In the manufacture of traveling wave tubes, the steps includingsubassemblingahelix and probes concentrically of a mandrel,.placing-saidrnandrel in a vitreous tube, fixing said probes in:relation to said vitreous tube to prevent elongation and.foreshortening of said helix, and heating said assembly while rotatingto shrink .said vitreous tube onto said helix to permanently fix1thehelix turns in place.

3. In the manufacture of traveling wave tubes, the steps includingsubassembling a helix .and probes "concentrically of a mandrel, placingsaid smandrel in a vitreous tube, fixing said probes to said vitreoustube to prevent elongation and foreshortening of said helix, drawing avacuum in said vitreous tube, axially rotating the assembly of-saidmandre'land vitreous tube, 'andheating said assembly whilerotating toatemperature sufil :cient to shrink said vitreous :tube at a low :fiowrate to uniformly engage the helix turns circumferentially and alongtheir length.

4. In the manufacture of .traveling wave tubes, the steps includingsubassembling a helix and probes sconic'entrically :of a mandrel,placing said mandrel in a vitreous tube, fixing said :probes to saidvitreous tube to prevent elongation and *foreshortening of said helix,drawing a vacuum of the 'order of one-tenth of an atmosphere in saidvitreous tube,'axi'ally rotating the assembly of said mandrel andvitreous tube, and heating said assembly while rotating to atemperaturesufficient toshrink said vitreous tube at a low flow' r'ateto uniformly. engage the helixturns circumferentially and along theirlength.

5. A method of constructing a helix assembly for a traveling wave tubeincluding an envelope of thermoplastic material having an enlarged bulband an elongated tube including the steps of placing the assembly of awire helix on a supporting mandrel Within said tube with said helix incontact with the inner wall of said tube, creating a pressuredifferential between the inside and outside walls of said tube, heatingsaid tube to shrinking temperature suflicient to soften saidthermoplastic material, and correlating the relationship between saidpressure differential and said shrinking temperature whereby saidthermoplastic material sags into substantially continuous embracingcontact both circumferentially and lengthwise of said wire and at a lowflow rate allowing for accurate control over and the limited penetrationof said thermoplastic material between the of said helix.

6. A method for constructing a helix assembly including the steps ofwinding wire in a helical configuration on a supporting mandrel,surrounding said wire by a snugly fitting vitreous envelope, creating apressure differential between the inside and outside of said envelope,heating said envelope to a shrinking temperature suflicient only tosoften said vitreous material, and correlating the relationship betweensaid pressure difierential and said shrinking temperature to cause saidvitreous material to sag into substantially continuous circumferentialembracing contact throughout the length of said wire and at a low fiowrate assuring accurate control over and the limited penetration of saidvitreous material between the respective turns of said wire.

7. A method for constructing a helix assembly including the steps ofwinding wire in a helical configuration on a supporting mandrel, heattreating said wire to set said Wire in said helical configuration,surrounding said wire by a snugly fitting vitreous envelope, creating apressure differential between the inside and outside of said envelope,heating said envelope to a shinking temperature respective turnssufficient only to soften said vitreous material, and correlating therelationship between said pressure difierential and said shrinkingtemperature to cause said vitreous material to sag into substantiallycontinuous circumferential embracing contact with said Wire throughoutthe length thereof and at a low flow rate assuring accurate control overand the limited penetration of said vitreous material between therespective turns of said wire.

8. A method for constructing a helix assembly including the steps ofwinding wire in a helical configuration on a supporting mandrel,surrounding said wire by a snugly fitting vitreous envelope, creating apressure dilferential between the inside and outside of said envelope,axially rotating the assembly of said supporting mandrel and en velope,heating said envelope to a shrinking temperature sufiicient only tosoften said vitreous material, and correlating the relationship betweensaid pressure ditferential and said shrinking temperature to cause saidvitreous material to sag into substantially continuous circumferentialcontact with said Wire throughout the length thereof and at a low flowrate assuring accurate control over and the limited penetration of saidvitreous material between the respective turns of said wire.

9. A method for constructing a helix assembly for a traveling wave tubeincluding an envelope having an enlarged bulb and an elongated tubeincluding the steps of winding a wire helix on a supporting mandrel,heat treating said helix to fix the turn distribution thereof,assembling said mandrel within said elongated tube with said helix incontact with the inner wall of said elongated tube, creating a pressuredifierential between the inside and outside walls of said helix, heatingsaid elongated tube to a shrinking temperature sufficient only to softensaid vitreous material while rotating the assembly, and correlating andmaintaining the relationship between said pressure differential and saidshrinking temperature to cause said vitreous material to sag intosubstantially continuous embracing contact both circumferentially andaxially of said helix and at a low flow rate assuring for accuratecontrol over and the limited penetration of said vitreous materialbetween the respective turns of said helix.

10. A method for constructing a helix assembly for a traveling wave tubeincluding an envelope an enlarged bulb and an elongated tube includingthe steps of winding a wire helix on a supporting mandrel, heat treatingsaid helix to fix the turn distribution thereof, assembling said mandrelwithin said elongated tube with said helix in contact with the innerwall of said elongated tube, fixing both said mandrel and said helix inrelation to said elongated tube, creating a pressure differentialbetween the inside and outside walls of said helix, heating saidelongated tube to a shrinking temperature sufficient only to soften saidvitreous material while rotating the assembly, and correlating andmaintaining the relationship between said pressure diiierential and saidshrinking temperature to cause said vitreous material to sag intosubstantially continuous embracing contact both circumfer-- entially andaxially of said helix at a low flow rate allowing for accurate controlover and the limited penetration of said virteous material between therespective turns of said helix.

References Cited in the file of this patent UNITED STATES PATENTS2,338,336 Koch Ian. 4, 1944 2,457,218 Ferrell Dec. 28, 1948 2,611,101Wallauschek Sept. 16, 1952 2,619,706 Vause Dec. 2, 1952 2,706,366 BestApr. 19, 1955 FOREIGN PATENTS 985,536 France Mar. 14, 1951

